The present invention relates to differentials, and more particularly, to the retention of the cross pin, on which pinion gears are rotatably disposed, therein.
Differentials are well known in the prior art and allow each of a pair of output shafts or axles operatively coupled to a rotating input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Typically, each of the axles is rotatably fixed to one of a pair of side gears, which are both intermeshed with a pair of first pinion gears. These pinion gears are rotatably disposed about opposite ends of a cylindrical cross pin which extends through diametrically opposite, holes in the circumferential wall of the rotating differential casing. The cross pin is fixed to the casing such that the first pinion gears revolve about the axis of rotation of the axles and side gears with the casing. As will be discussed further hereinbelow, typically, one end of the cross pin is provided with a cross bore which is aligned with holes in the casing; a bolt extends through the casing holes and the cross bore to retain the cross pin to the casing.
The casing is typically provided with a ring gear attached about its outer periphery, and which is intermeshed with a second pinion gear which is drivingly rotated by an engine. The cross pin, which is caused to rotate with the casing, imparts a driving force on the first pinion gears, the teeth of which impart a driving force on the teeth of the side gears intermeshed therewith. Hence, rotation of the axles, which are coupled to each other through the side gears and first pinion gears, is achieved. During differentiation, there is relative movement between the first pinion gears and the side gears, and the axles rotate at different speeds. Thus, a differential distributes the torque provided by the input shaft between the two axles and their respective driven wheels.
The completely open differential, i.e., a differential without clutches or springs which restrict relative rotation between the axles and the rotating differential casing, is not well suited to slippery conditions in which one driven wheel experiences a much lower coefficient of friction than the other driven wheel: for instance, when one wheel of a vehicle is located on a patch of ice and the other wheel is on dry pavement. Under such conditions, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a "spin out" of that wheel. Since the maximum amount of torque which can be developed on the wheel with traction is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. A number of methods have been developed to limit wheel slippage under such conditions.
Prior means for limiting slippage between the axles and the differential casing use a frictional clutch mechanism, either clutch plates or a frustoconical engagement structure, operatively located between the rotating case and the axles. Certain embodiments of such prior means provide a clutch element attached to each of the side gears, and which frictionally engages a mating clutch element attached to the rotating casing or, if the clutch is of the conical variety, a complementary interior surface of the casing itself. Such embodiments may also include a bias mechanism, usually a spring, to apply an initial preload between the clutch and the differential casing. By using a frictional clutch with an initial preload, a minimum amount of torque can always be applied to a wheel having traction, e.g., a wheel located on dry pavement. The initial torque generates gear separating forces between the first pinion gears and the side gears intermeshed therewith. The gear separating forces urge the two side gears outward, away from each other, causing the clutch to lightly engage and develop additional torque at the driven wheels. Examples of such limited slip differentials which comprise cone clutches are disclosed in U.S. Pat. Nos. 4,612,825 (Engle), 5,226,861 (Engle), 5,556,344 (Fox), and 5,989,147 (Forrest et al.), issued Nov. 23, 1999, all of which are assigned to the assignee of the present invention and expressly incorporated herein by reference.
Certain prior art limited slip differentials provide, between the first of the two side gears and its associated clutch element, interacting camming portions having ramp surfaces. In response to an initiating force, this clutch element is moved towards and into contact with the surface against which it frictionally engages, which may be a mating clutch element attached to the casing, or an interior surface of the casing itself, as the case may be, thereby axially separating the clutch element and its adjacent first side gear, the ramp surfaces of their interacting camming portions slidably engaging, the rotational speed of the clutch element beginning to match that of the differential casing due to the frictional engagement. Relative rotational movement between the ramp surfaces induces further axial separation of the clutch element and the first side gear. Because the clutch element is already in abutting contact with the surface against which it frictionally engages, the first side gear is forced axially away from the clutch element by the camming portions.
A transfer block element disposed about the cross pin, between the pinion gears disposed thereon, is provided to transfer axial movement from the first side gear to the second side gear, which is disposed on the opposite side of the cross pin. The transfer block element is allowed to move laterally relative to the cross pin, along the axis of the axles. The transfer block element is abutted by the axially moving first side gear and is forced into abutment with the second side gear, to which is rotatably fixed a second clutch element which also operatively engages the rotating casing, thereby providing additional clutched engagement between the clutch elements and the casing. The following example, which describes a previous limited slip differential having first and second cone clutches and an electromagnetic initiating force, is illustrative:
FIG. 1 depicts differential 10 which comprises rotatable casing 12 constructed of joined first and second casing parts 12a and 12b, respectively, and providing inner cavity 14, which is defined by the interior surface of the circumferential wall portion of first casing part 12a and end wall portions 16, 18 of first and second casing parts 12a, 12b, respectively. Disposed within cavity 14 are side gears 20, 22 and pinion gears 24, 26. The teeth of the side gears and pinion gears are intermeshed, as shown. Pinion gears 24, 26 are rotatably disposed upon cylindrical cross pin 28, which extends along axis 30. Cross pin 28 is made of a suitable material such as, for example, heat treated 8620 steel. The ends of cross pin 28 are received in holes 32, 34 diametrically located in the circumferential wall of casing part 12a. One end of cross pin 28 is provided with cross bore 36, which is aligned with holes 38, 40 in casing part 12a, as shown. Bolt 42 extends through hole 38, cross bore 36 and hole 40 to retain the cross pin in its proper position relative to casing 12. Portion 44 of bolt 42 is provided with threads which are engaged with hole 38.
Axles 46, 48 are received through hubs 50, 52, respectively formed in casing end wall portions 16, 18, along common axis of rotation 54, which intersects and is perpendicular to axis 30. Axles 46, 48 are respectively provided with splined portions 56, 58, which are received in splines 60, 62 of side gears 20, 22, thereby rotatably fixing the side gears to the axles. The axles are provided with circumferential grooves 64, 66 in which are disposed C-rings 68, 70, which prevent the axles from being removed axially from their associated side gears. Casing part 12a is provided with a large aperture (not shown) located in the circumferential wall thereof, between holes 32, 34, for assembly and service access to C-rings 68, 70. Terminal ends 72, 74 of the axles may abut against the cylindrical surface of cross pin 28, thereby restricting the axles' movement toward each other along axis 54.
Clutch element 76 is attached to side gear 20 and rotates therewith. Clutch element 76 is of the cone clutch variety and has frustoconical surface 78 which is adjacent to, and clutchedly interfaces with, complementary surface 80 provided on the interior of casing part 12a. Clutch element 82 is also of the cone clutch variety and has frustoconical surface 84 which is adjacent to, and clutchedly interfaces with, complementary surface 86 also provided on the interior of casing part 12a. Clutch element 82 is provided with annular surface 88 which faces annular surface 90 of side gear 22. Surface 88 is provided with a plurality of circumferentially-aligned arcuate grooves 92. Grooves 92 are provided with surfaces which ramp "upwards" toward surface 88 one circumferential direction along the groove. Similarly, surface 90 is provided with an equal plurality of circumferentially-aligned arcuate grooves 94 having surfaces which ramp "upwards" toward surface 90, but in an opposite circumferential direction. Disposed in each opposed pair of grooves 92, 94 is ball 96. Hence, grooves 92, 94 and balls 96 comprise a type of interacting camming mechanism well-known in the art as a ball ramp arrangement. Briefly, relative rotation between clutch element 82 and side gear 22 imparts axial separation therebetween as balls 96 ride up on the ramp surfaces of grooves 92 and 94. Alternatively, a surfaces 88, 90 may be provided with interacting cam surfaces (not shown) which project therefrom and have slidably engaging ramp surfaces which axially separate clutch element 82 and side gear 22 as they rotate relative to one another; this type of camming mechanism, too, is well known in the art. Balls 96 are urged into the deepest portions of grooves 92, 94, and surfaces 88, 90 brought into their closest proximity to each other, by means of Belleville spring 98, which is disposed between surface 100 of clutch element 82 and snap ring 102 received in circumferential groove 104 provided in portion 106 of side gear 22.
Provided on the exterior surface of casing part 12a is flange 108, to which a ring gear (not shown) is attached. The teeth of the ring gear are in meshed engagement with the teeth of a pinion gear (not shown) which is rotatably driven by an engine (not shown), thus rotating differential case 12 within an axle housing (not shown) from which axles 46, 48 project. As casing 12 rotates, the sides of holes 32, 34 bear against the portions of the cylindrical surface of cross pin 28 in the holes. The rotation of cross pin 28 about axis 54 causes pinion gears 24, 26 to revolve about axis 54. The revolution of the pinion gears about axis 54 causes side gears 20, 22 to rotate about axis 54, thus causing at least one of axles 46, 48 to rotate about axis 54.
Electromagnet 110 is rotatably fixed relative to the axle housing (not shown) in which differential 10 is disposed, and is supported on casing portion 12b by bearing 112. The voltage applied to electromagnet 110 may be controlled by a control system (not shown) which is in communication with sensors (not shown) which indicate excessive relative rotation between axles 46, 48. Electromagnet 110 is disposed in close proximity to casing 12, which rotates relative thereto. As the electromagnet is energized, an initiating force is applied to clutch element 82 by a toroidal electromagnetic flux path (not shown) which is established about the annular electromagnet; the flux path flows through ferrous casing portions 12a and 12b and through clutch element 82. Clutch element 82 is thus magnetically drawn into engagement with the casing during operation of the electromagnet.
As shown in FIG. 1, during normal differential operation, with electromagnet 110 deactivated, surfaces 88 and 90 of clutch element 82 and side gear 22, respectively, are closely adjacent and slightly separated. Balls 96 are urged into the deepest portions of slots 92, 94 by Belleville spring 98 and by gear separation forces between side gear 22 and pinion gears 24, 26. As viewed in FIG. 1, Belleville spring 98 urges cone clutch element 82 rightward, axially away from snap ring 102, and the gear separation forces urge side gear 22 leftward, toward clutch element 82.
As electromagnet 110 is activated, further axial separation of cone clutch element 82 and side gear 22 is induced as cone clutch element 82 is magnetically pulled to the left, against the force of Belleville spring 98, into clutched engagement with casing part 12 through mating frustoconical surfaces 84, 86; side gear 22 temporarily maintains its axial position. As cone clutch element 82 and side gear 22 separate axially, balls 96 are caused to rotate along the ramping paths of slots 92, 94 due to the relative rotation between cone clutch element 82, which is in frictional engagement with the case, and side gear 22; the rotation of the balls along the slots induces yet further axial separation of cone clutch element 82 and side gear 22, the side gear urged rightward as viewed in FIG. 1, its surface 114 abutting adjacent surface 116 of transfer block element 118.
Transfer block element 118 is disposed about cross pin 28, and held in position along the cross pin by its opposite ends abutting pinion gears 24, 26. Transfer block 118 moves laterally relative to the cross pin, along axis 54, such that rightward movement of side gear 22, described above, is transferred to side gear 20. Surface 120 of transfer block 118 is brought into abutting contact with surface 122 of side gear 20. Thus, during actuation of electromagnet 110, side gear 22 is urged rightward, as viewed in FIG. 1, into abutting contact with transfer block element 118, which may be made of steel. Transfer block element 118 moves rightward, into abutting contact with side gear 20; and side gear 20 moves rightward, urging surface 78 of clutch element 76 into frictional engagement with surface 80 of case part 12a, thereby providing additional torque transfer capacity to the differential than would otherwise be provided with single cone clutch element 82.
In use, the circumferential wall of casing 12 experiences a substantial amount of stress, the entirety of the energy transferred from the engine to the axles being communicated from the rotating casing through its holes 32, 34 bearing on the cylindrical surface at opposite ends of the cross pin. In circumstances where an extraordinary amount of stress is exerted on casing 12, damage thereto may occur. As mentioned above, cross pin 28 is secured to casing part 12a by removable, partially threaded bolt 42 which extends into aligned holes 38, 40 in casing part 12a. Holes such as holes 38, 40, placed near the interface of the casing and the cross pin may compromise the strength of the casing. Further, cross bore 36, which extends through one end of cross pin 28, may compromise the strength of the cross pin. It is desirable to eliminate holes such as 38, 40, in the casing wall, and cross bores such as 36 in the ends of the cross pin, which are subject to high shear stresses.
Further, in particular circumstances, bolt 42 may back out of its threaded engagement in casing hole 38, and fall out of casing holes 38, 40 and cross pin cross bore 36, causing cross pin 28 to dislodge from its position within aligned bores 32, 34 in casing part 12a, resulting in complete failure of the differential mechanism. Such a failure renders the vehicle in which differential 10 is installed inoperable. Bolt 42 may be caused to back out of its threaded engagement by continuous vibrations or strains placed on the casing forces during normal operation of differential 10. A more effective means of retaining the cross pin in aligned bores 32, 34 is thus desirable.
Bolt 42 is also disposed in a somewhat inconvenient location for service purposes which require removal of the cross pin while the differential is installed in the axle housing. Because bolt 42 is rather long and is disposed such that it must be removed along a line parallel with axis 54, access to and removal of the bolt while the differential is installed in the axle housing may be hindered. A more accessible means of detachably securing the cross pin to the differential is therefore desirable.
Thus, what is needed is a means of retaining the cross pin of a limited slip differential which provides greater casing strength and easier accessibility to the fastener which retains the cross pin to the casing.